Presently there are several different types of procedures that can be used for performing refractive surgery on the cornea of an eye to correct vision defects. In general, these procedures can be categorized according to the kind of instrument that is used to perform the surgery (e.g. a “mechanical means” or a “laser means”). Further, within the “laser means” category, there are essentially two methodologies for altering stromal tissue that differ from each other depending on whether tissue is actually removed by photoablation, or is merely weakened by a phenomenon commonly referred to as Laser Induced Optical Breakdown (LIOB).
A removal of tissue from the cornea of an eye, when using a laser means to correct a vision defect, is typically accomplished by the photoablation of exposed corneal tissue. For example, the well-known procedures of LASIK and PRK (Photo Refractive Keratectomy) both rely on the removal of exposed tissue by photoablation. On the other hand, it is also known that tissue inside the stroma can be merely weakened to correct a vision defect. For example, the weakening of intrastromal tissue for the purpose of correcting a vision defect is disclosed and claimed in U.S. patent application Ser. No. 11/958,202 for an invention entitled “Method for Intrastromal Refractive Surgery” which is assigned to the same assignee as the present invention. More specifically, the result of weakening tissue is a redistribution of biomechanical stresses in the stroma that responds to Intra-Ocular Pressure (IOP) to reshape the cornea. It can happen, however, that neither the removal of corneal tissue, nor the weakening of stromal tissue, when performed alone, may be able to achieve the desired refractive result. Much here depends on the cause of the vision defect that needs to be corrected. Thus, there are limits to the extent of an effective refractive correction when either a weakening of stromal tissue or an actual removal of tissue are considered separately.
The inherent limitations of tissue removal procedures are primarily a result of the amount of corneal tissue that can be removed. Specifically, for the rather common visual defects of myopia, hyperopia and astigmatism, it is known that going beyond the limits set forth below can cause unwanted instabilities in the cornea. In general, the practical limits for effective refractive corrections to be achieved for the more common visual defects, by tissue removal alone, are:
MyopiaPRK−6 diopter limitLASIK−8 diopter limitHyperopiaPRK+3 diopter limitLASIK+5 diopter limitAstigmatismPRK 3 diopter limitLASIK 5 diopter limitAlso, it is known that presbyopia, in combination with the visual defects considered above, may require a refractive correction that goes beyond the limits set forth above.
On the other hand, when stromal tissue is weakened, rather than removed, refractive corrections are limited to approximately 2 or 2.5 diopter. Further, for safety reasons, any stromal tissue that is to be weakened should not be within the one hundred microns immediately anterior to Descemet's membrane and the endothelium.
With the above in mind, it is an object of the present invention to provide a system and method for making myopic/hyperopic/astigmatic corrections requiring refractive changes that go beyond the limits provided by only removing corneal tissue. Another object of the present invention is to provide a system and method for combining corneal tissue removal with an intrastromal redistribution of biomechanical stresses to achieve a predetermined refractive correction for an eye. Still another object of the present invention is to provide a system and method for both removing and weakening corneal tissue, in combination, to provide refractive corrections for an eye that are relatively easy to implement and comparatively cost effective.